:t zがダメだけど、D (X "123") 123 (1::Y)やD (X "123") 123 (X "456")ではデータ構築可能

Test2.hs

{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE GADTs #-}
module Test where

data X = X String 
       deriving Show
                
type Y = Int

class Z a where
instance Z X
instance Z Y

data D = forall a. (Show a, Z a) => D { x :: X
                                      , y :: Y
                                      , z :: a
                                      }  
{- GHCでは使えないけど、これと同じ。

data D = D { x :: X
           , y :: Y
           , z :: exists a. (Show a, Z a) => a
           }
-}

test = case D (X "Hello") 123 (456 :: Y) of
         D x y z -> print z